In 3GPP (3rd Generation Partnership Project), the packet-switched communication systems LTE (Long Term Evolution) and HSPA (High Speed Packet Access) have been specified for wireless transmission of data packets between user terminals and base stations in a cellular/mobile network. LTE and other systems generally use OFDM (Orthogonal Frequency Division Multiplexing) involving multiple closely spaced orthogonal sub-carriers, which is a well-known technique in the art. The sub-carriers are further divided into timeslots where each frequency/timeslot combination is referred to as a resource element.
In this description, a “sending node” is a node which sends information carrying signals over a wireless link, and a “receiving node” is a node which receives and hopefully detects those signals. In the case of downlink transmissions, the sending node is a base station and the receiving node is a user terminal, and vice versa for uplink transmissions.
The use of multiple antennas in signal sending nodes and/or signal receiving nodes may enhance capacity, coverage and reliability in a wireless communication system, e.g. by achieving increased data throughput and/or better signal detection at the receiving node. Multiple antennas can be employed in both user terminals and base stations to enable parallel and spatially multiplexed data streams using the same radio channel resource, which is commonly referred to as “MIMO” (Multiple-Input, Multiple-Output).
In particular, LTE is currently being developed to support and utilise MIMO related techniques to provide high data rates in favourable channel conditions. Other wireless communication systems that may also be relevant for the following description include WCDMA (Wideband Code Division Multiple Access), WiMAX, UMB (Ultra Mobile Broadband), GPRS (General Packet Radio Service) and GSM (Global System for Mobile communications).
In MIMO systems, spatial multiplexing is obtained by transmitting plural parallel information carrying signals at the same time and frequency while the different signals are spatially separated from each other by means of plural transmit antennas. The number of parallel signals or data streams being transmitted simultaneously is referred to as the “transmission rank”.
Furthermore, by adapting the signal transmission to the current channel conditions or properties, significant improvements may be achieved. “Transmission rank adaptation” is one form of such adaptation where the transmission rank is dynamically adjusted to what the used channel is currently able to support. Another form of signal transmission adaptation is “channel dependent precoding” where the phases and amplitudes of multiple parallel signals are adjusted to match the current channel properties. Classical beam-forming is in fact one example of precoding where the signal phase of the signal from each transmit antenna is adjusted such that the signals add constructively at the receiving node.
The parallel signals to be transmitted from multiple antennas form a vector-valued signal. When precoding is employed, the signals are effectively adjusted at the sending node by multiplying the vector-valued signal by a selected precoder matrix. This procedure is illustrated schematically in FIG. 1. A coding and modulation unit, not shown, takes information bits as input and basically produces a sequence of information carrying symbol vectors in different parallel symbol streams referred to as “layers” 100. In FIG. 1, r different layers 1-r are shown implying a transmission rank of r. Thus, the signal to be transmitted is comprised of r parallel symbol streams forming elements of a symbol vector s, which are fed into a precoder unit 102.
In precoder unit 102, an NT×r precoder matrix W is currently used to adjust the r symbol streams in vector s, where NT denotes the number of transmit antennas or antenna ports used. The r symbols in the symbol vector s are thus multiplied by the NT×r precoder matrix W to produce an adjusted symbol vector s′ which is converted into OFDM signals by IFFT (Inverse Fast Fourier Transform) units 104 in a well-known manner. The produced OFDM signals are then finally transmitted from the antennas or antenna ports 1−NT.
An NT×NR MIMO channel H, used between a sending node with NT transmit antennas and a receiving node with NR receive antennas, is generally represented with the NT×NR matrix, and the precoder matrix W is often chosen to match the properties and characteristics of channel H. When signals are transmitted over channel H, a received vector yk for a certain resource element on a sub-carrier k, or alternatively a resource element k, can be modelled by the receiving node as:yk=HWsk+ek  (1)
assuming no inter-cell interference and that precoder matrix W is known. The term ek is modelled as a noise vector obtained by realisations of a random process.
In channel dependent precoding, the precoder matrix may be selected at the sending node based on information on the current channel properties as reported from the receiving node to the sending node in a feedback report. A common approach is to select the precoder matrix from a predefined set of precoder matrices, referred to as a codebook which is known at both nodes. Codebook based precoding is generally employed by the LTE standard.
The receiving node, typically a user terminal, detects the current channel properties based on measurements on signal transmissions from the sending node, typically a base station, and evaluates the precoder matrices in the codebook to determine the most appropriate one for use in the current conditions. The receiving node reports back to the sending node a precoder matrix that is recommended for signal transmission, and the sending node is then able to apply a suitable precoder matrix for the transmission, taking the recommended one into consideration.
The receiving node may recommend a single precoder matrix assumingly covering a relatively large bandwidth of multiple sub-carriers allocated for the used channel, i.e. “wideband” precoding. Alternatively, when the channel properties are notably different for different frequencies, it may be beneficial to match individual frequencies of the channel and provide a frequency-selective precoding recommendation, specifying different precoders for different sub-carriers or sub-bands of the total bandwidth used.
As a result from the above, channel dependent precoding typically requires substantial signalling support, particularly for frequency-selective precoding schemes. In addition to the above-described feedback signalling from the receiving node to the sending node, signalling in the opposite direction is typically also necessary to indicate which precoder is actually used in the signal transmission. Hence, the sending node may not be assured it has obtained a correct or relevant precoder report from the receiving node, and the receiving node must also make sure which one is used in order to process the received signals correctly.
The amount of signalling between the receiving node and the sending node can be reduced if the sending node merely sends a brief precoder confirmation, indicating whether the recommended precoder(s) has been applied or not. Basically, a single bit can be used for this purpose, where “1” could mean that the transmitter has applied the recommended precoder(s), while “0” could mean that another, possibly fixed or default precoder is used, thereby overriding the precoder recommendation. For example, “0” would also be signalled if the feedback information could not be correctly decoded at the sending node.
However, the above precoder confirmation scheme implies that any decoding errors in the feedback information should preferably be detected, and the feedback information must therefore be coded accordingly, e.g. by including a CRC (Cyclic Redundancy Check) in the report, which increases the report size even more. An alternative to using a fixed or default precoder scheme is to also signal a single wideband precoder to the receiving node. Other precoder confirmation scheme have also been proposed, which are not necessary to described here though. Instead of explicitly signalling to the receiving node which frequency-selective precoders are actually used by the sending node, the above precoder confirmation methods can thus be employed to substantially reduce the amount of overhead signalling to the receiving node.
The encoded bits, or even modulated symbols, originating from a particular block of information bits, often called a transport block, can be referred to as a “codeword”. This term is also used in LTE to describe the output from a specific so-called “HARQ (Hybrid Automatic Repeat ReQuest) process” serving a particular transport block and providing for retransmission of any erroneously decoded codeword. A HARQ process involves various coding schemes such as turbo encoding, rate matching, interleaving etc.
A generated codeword is modulated and distributed over the antennas of the sending node. Further, data from plural codewords can be transmitted simultaneously, also known as “multi-codeword transmission”. For example, in a sending node with four transmit antennas 1-4, a first modulated codeword may be mapped to antennas 1 and 2, and the next codeword may be mapped to antennas 3 and 4, and so forth. In the above context of precoding, the codewords are first mapped to layers instead of being mapped directly to the antennas.
In the field of multi-antenna transmissions of high data rate, a specific feature of the prevailing channel conditions/properties is the so-called “channel rank” which indicates how many simultaneous signals or data streams that the current channel can actually support. Basically, the channel rank can vary from one up to the least number of transmit and receive antennas present at the sending and receiving nodes, respectively. For example, in a 4×2 MIMO system with four transmit antennas and two receive antennas, the maximum channel rank is two.
Furthermore, the channel rank may vary in time, e.g. since fluctuating parameters such as fast fading and interference typically influence the channel properties. Moreover, the channel rank determines how many layers, and ultimately also codewords, that can be successfully transmitted simultaneously. Hence, if the current channel rank is only one when simultaneously transmitting two codewords mapping to two separate layers, i.e. using a transmission rank of two, the two signals corresponding to the codewords will most likely interfere so much that both codewords are erroneously detected at the receiving node.
When precoding is employed, the transmission can be adapted to the channel rank by using as many layers as the current channel rank. In a simple case, each layer is transmitted over a particular antenna. However, the number of codewords to transmit may differ from the number of layers used, e.g. as in LTE. In that case, the codewords must be mapped onto the layers. For example, when four transmit antennas are available at the sending node, the maximum number of codewords is limited to two, while up to four layers can be transmitted simultaneously over the respective antennas when the current channel rank=4. A fixed channel rank-dependent mapping of codewords onto layers could then be used.
FIGS. 2a-e illustrate some examples of possible mapping of codewords onto layers for different channel ranks, and when four transmit antennas are available at a sending node. In the figures, “S/P” denotes an operation of transforming serial signals to parallel signals, which is well-known in the art. In these examples, the produced layers are distributed on the four antennas 202 by a precoder unit 200, which adjusts the symbol streams in the layers by means of a selected precoder matrix basically as described above.
In FIG. 2a where rank=1, one codeword CW1 is mapped onto a single layer L1. In FIG. 2b where rank=2, a first codeword CW1 is mapped onto a first layer L1, while a second codeword CW2 is mapped onto a second layer L2. In FIG. 2c where rank=2 again, one codeword CW1 is alternatively mapped onto two layers L1 and L2. In FIG. 2d where rank=3, a first codeword CW1 is mapped onto a first layer L1, while a second codeword CW2 is mapped onto a second layer L2 and a third layer L3. In FIG. 2e where rank=4, a first codeword CW1 is mapped onto a first layer L1 and a second layer L2, while a second codeword CW2 is mapped onto a third layer L3 and a fourth layer L4.
When dynamic transmission rank adaptation and channel dependent precoding are employed for a MIMO channel, substantial amounts of MIMO related control information needs to be signalled from the sending node to the receiving node to support the precoding, as mentioned above. In LTE for example, a control channel called PDCCH (Physical Downlink Control Channel) is used for conveying such MIMO related information from a signal sending base station to a signal receiving user terminal. The PDCCH is currently configured with various information fields in which 16 bits are used for MIMO information, out of which 8 bits relate to precoding.
However, it is a drawback in the existing ways of conveying MIMO and precoder related information from a sending node to a receiving node, as exemplified by the above-mentioned PDCCH, that a large signalling overhead is required. As a result, the coverage of control signalling may be seriously reduced, implying that control channel coverage may well be a limiting factor in the use of MIMO. Furthermore, no efficient support has been provided for so-called transmission rank override in the above-described precoder confirmation.
Transmission rank override thus means that the sending node is able to override the recommendation of transmission rank obtained from the receiving node. This functionality may be useful in several situations, such as when the buffer has very limited amounts of data to send, or when scheduling on a significantly smaller part of the bandwidth than what the single recommended “wideband” transmission rank refers to, etc. In addition, when a HARQ process is originally transmitted mapped to two layers using transmission rank 3 or 4, that HARQ process cannot efficiently provide for retransmission if the transmission rank is reduced to 2 due to fluctuating channel properties.
Hence, it is generally a problem that the payload for conveying MIMO and precoder related information, e.g. transmission rank indicator (TRI), precoder confirmation, explicit precoder matrix indicator (PMI), from a signal sending node to a signal receiving node is typically large and requires substantial signalling bandwidth. Conversely, when only a limited given payload size is available for control signalling, there may be a need to support the signalling of additional control information.